This invention relates generally to over-voltage protection circuits and, more particularly, to circuits for preventing excessive voltage build-up at the output of an electric machine when an electrical load is removed therefrom.
An electric generating machine such as a three phase alternator provides an output voltage that is dependent upon the field excitation provided to the machine and to the amount of current being drawn by the load connected to the machine. In general, the amount of excitation current required to provide a predetermined output voltage is proportional to the amount of current being drawn by the load. Voltage regulating circuitry may be provided to adjust the excitation current to maintain the output voltage substantially constant for various load conditions. Such voltage regulator circuits are effective to maintain the output voltage substantially constant for relatively slowly varying load conditions, however, for rapidly varying loading conditions, output voltage transients cannot be prevented by controlling the excitation current. For example, when the battery of an automobile has been disconnected from an automotive alternator, resulting in a loss in the filtering provided thereby, and subsequently a large power consuming accessory such as an air conditioner is turned off, large voltage transients can occur at the output of the alternator due to the magnetic energy stored in the field even though the voltage regulator has terminated the field excitation.
Several techniques for eliminating the voltage transients resulting from a rapid change in the electrical load on a generating machine are known. Such techniques generally employ circuitry for sensing the output voltage of the machine and for shunting or otherwise isolating the output windings of the machine when an over-voltage condition is sensed. Representative systems are described in U.S. Pat. Nos. 3,488,560, issued Jan. 6, 1970, to R. L. Konopa, and 3,581,150, issued May 25, 1971, to T. E. Kirk et al.
Whereas these techniques provide a way to effectively suppress an over-voltage transient, in a poly-phase machine having several windings, the prior art techniques require a separate switching circuit for each phase winding of the machine. The separate switching circuits result in relatively complex circuitry which is expensive and prone to failure as a result of the complexity thereof.
Accordingly, it is an object of the present invention to provide a simplified over-voltage protection circuit for a wye connected electric generating machine.
It is another object of the present invention to provide a shunt type over-voltage protection system for a multiple phase generating machine that requires only a single power switching element.
In accordance with a preferred embodiment of the invention, the phases of the power generating machine are connected together in a wye configuration, and rectifier means are provided for connecting the windings to a pair of output terminals of the machine. An electronic switching circuit comprising a silicon controlled rectifier is connected to the common wye terminal of the machine and to one of the output terminals, generally the ground or common terminal. A voltage sensing circuit including a Zener or avalanche diode is connected between the common wye terminal and a control terminal of the silicon controlled rectifier to render the silicon controlled rectifier conductive to shunt the wye terminal to the output terminal when the voltage at the wye terminal exceeds a predetermined level. The voltage sensing Zener diode is also connected to field energizing circuitry for removing the excitation current from the field during the over-voltage condition.
The invention together with the above and other objects and advantages may be best understood from the following detailed description of the embodiments of the invention shown in the accompanying drawing, where: